The invention relates to a material handling system, and particularly a method of operating a material handling system having a hoist or lifting apparatus.
It is known to use an alternating current (AC) variable frequency drive or power supply on a material handling system. The AC variable frequency drive provides power to one or more motors of a hoist or lifting apparatus. Prior systems typically use a feedback system from an encoder or tachometer to insure the motor is following a commanded speed within a predetermined window. If the motor is not following the commanded speed, then it is assumed the motor is not producing enough torque to control the load at that speed, and a brake will set. Therefore, the encoder feedback system provides an important feature for determining load integrity or stability. The encoder feedback system is particularly useful at low frequencies (e.g., at frequencies below the motor slip speed) where the hoist is most susceptible to losing load integrity.
One problem with a material handling system having an encoder feedback system is that the encoder of the feedback system is attached to the hoist motor. Attaching the encoder to the motor results in more maintenance for the handling system than if no encoder is present. Additionally, an added encoder requires an extra cable between the motor and the AC drive. Eliminating the encoder feedback system reduces potential downtime due to encoder damage or malfunction, reduces potential service costs, and reduces the number of cables between the motor and the drive. Therefore, it is desirable to have a material handling system that verifies load integrity, and prevents possible load loss without the use of an encoder or similar feedback device.
Accordingly, the invention provides a material handling system including a lifting apparatus operable to lift a load, an alternating current (AC) motor connected to the lifting apparatus, a brake connected to the lifting apparatus and being operable to prevent movement of the load, an inverter electrically connected to the AC motor and being operable to generate a current capable of driving the motor, a current sensor operable to sense a current value of the inverter signal, and a controller. The controller is operable to determine an applied torque producing current value, to determine a modeled torque producing current value, and to generate a brake-control signal if the applied torque producing current value varies from the modeled torque producing current value by a deviation amount.
The invention further provides a material handling system including a rotatable drum, an alternating current (AC) motor coupled to the rotatable drum, a brake coupled to the rotatable drum and being operable to prevent the rotatable drum from rotating, and an inverter electrically connected to the AC motor. The inverter is operable to generate an inverter signal having a frequency and a voltage. The material handling system further includes a current sensor operable to sense a current value of the inverter signal, and a controller. The controller is operable to determine an actual motor speed, to determine a modeled motor speed, and to generate a brake-control signal if the actual motor speed varies from the modeled motor speed by a deviation amount.
The invention further provides a method of controlling a material handling system that lifts a load. The method includes storing a model of a motor in a drive, and generating a signal in the drive. The signal has a voltage and a frequency. The method further includes providing the signal to the motor, sensing a current value of the signal, determining a modeled value based in part on the sensed current value, comparing an actual value to the modeled value to determine whether the load is stable, and generating an output that sets the brake when the load is potentially unstable.
In another embodiment, the method includes providing a model of a motor, generating a signal in a drive, providing the signal to the motor, and sensing a current of the signal. The sensing occurs within the drive. The method further includes determining a modeled torque producing current based in part on the sensed current, determining a modeled motor speed based in part on the sensed current, and determining an applied torque producing current, and determining an actual motor speed. The method further includes calculating a first difference value between the applied torque producing current and the modeled torque producing current, comparing the first difference with a first deviation amount, setting the brake when the first difference value is greater than the first deviation amount, calculating a second difference value between the actual motor speed and the modeled motor speed, comparing the second difference value with a second deviation amount, and setting the brake when the second difference value is greater than the second deviation amount.
Other features and advantages of the invention will become apparent from the detailed description and accompanying drawings.